1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and a semiconductor device. More specifically, the present invention relates to a semiconductor device and a method for manufacturing thereof including a method for forming a gate insulating film.
2. Background Art
Generally as a gate insulating film in a semiconductor device, a thermal oxide film is often formed on an Si substrate; however, during the formation of the thermal oxide film, a slight structural defect may occur in the vicinity of the boundary between the thermal oxide film and the Si substrate. The structural defect generates a fixed charge in an oxide film close to the Si substrate, or interface states derived from dangling bonds in the vicinity of boundary between the thermal oxide film and the Si substrate. Since the interface states are generally considered to cause degradation of properties of the semiconductor device, the interface state density must be lowered. Therefore, a method for improving the state of the boundary between the thermal oxide film and the Si substrate by annealing at a relatively high temperature after the formation of the thermal oxide film has been studied.
On the other hand, concurrent with the high densification and down-sizing of semiconductor devices in recent years, the thickness of gate insulating films of transistors has also been reduced. When the thickness of a gate insulating film is reduced, leakage current increases to a considerable value in a conventional gate insulating film consisting of SiO2. In order to inhibit increase in leakage current, a method for nitriding the surface of an SiO2 film to convert the surface of the gate insulating film to an oxynitride film has been adopted. Particularly in a p-MOS, the use of such a nitriding oxide film can prevent the punch-through of boron in the gate electrode.
An example of methods for introducing nitrogen in an oxide film to form a gate insulating film is a method using heat treatment in a nitrogen-containing atmosphere, such as NO, N2O and NH3, after the formation of the thermal oxide film, and thereby, the surface of the thermal oxide film can be nitrided. According to this method, the surface of an SiO2 film can be nitrided to prevent leakage current, and the state of the boundary between the Si substrate and the thermal oxide film can be improved by heat treatment. Another method is a method for performing plasma nitriding after forming the thermal oxide film, and adding nitrogen to the surface of the thermal oxide film.
When the method for nitriding a thermal oxide film in an NO atmosphere or the like is used as described above, the temperature for introducing nitrogen is relatively high. If such a high temperature is used, nitrogen may be re-diffused to the vicinity of the boundary. If nitrogen is present in the vicinity of the boundary between the thermal oxide film and the Si substrate, the nitrogen causes an undesirable rise of interface states. Particularly in NBTI (negative bias temperature instability) state, the nitrogen distribution in the gate insulating film influences device characteristics significantly, causing the degradation of the device characteristics.
When plasma nitriding is used, since nitriding proceeds while substituting oxygen in a thermal oxide film with nitrogen, nitriding at a relatively low temperature is possible. However, in the boundary between the thermal oxide film and the Si substrate, a relatively poor boundary is easily formed causing the elevation of the interface state.
When devices having structures consisting of different thickness of gate insulating films are simultaneously formed, the application and peeling of a resist on and off the substrate must be repeated. Therefore, the boundary of the gate insulating film is significantly damaged, elevating the density of interface states.
In recent years, the use of a high-dielectric-constant film as a gate insulating film has been studied for reducing power consumption and preventing leakage current. In this case, an interfacial gate insulating film is formed between the high-dielectric-constant film and the Si substrate, and the interfacial gate insulating film is often a very thin film referred to as an IFL (interfacial layer). Particularly in the case of such a thin film, since the proportion of the boundary in the film is large, the effect of the interface states becomes larger, and the formation of a thin film of a higher reliability is required for reducing the density of interface states.